jackknife vs pocket knife

jackknife and pocket knife both are nouns.

Word	Noun	Adjective	Verb	Adverb
jackknife	Yes	No	Yes	No
pocket knife	Yes	No	No	No

As nouns, pocket knife is a hypernym of jackknife; that is, pocket knife is a word with a broader meaning than jackknife:

jackknife: a large knife with one or more folding blades
pocket knife: a knife with a blade that folds into the handle; suitable for carrying in the pocket

Other hypernyms of jackknife include pocketknife.

jackknife (noun)	pocket knife (noun)
a dive in which the diver bends to touch the ankles before straightening out	a knife with a blade that folds into the handle; suitable for carrying in the pocket
a large knife with one or more folding blades

jackknife (verb)	pocket knife (verb)
dive into the water bending the body at the waist at a right angle, like a jackknife